1. Field of the Invention
This invention relates to the generation of magnetic fields.
2. Related Art
The present invention has arisen as a result of a requirement in the field of crystal growth, but the application of the invention is not limited to such a field. Large single crystals of semiconductor material are grown by means of an apparatus which usually includes a crucible mounted coaxially within a heating element. The crucible contains a molten mass of the semiconductor material. The crucible can be raised or lowered relative to the heating elements and electrical energy for the heating elements is applied under computer control or otherwise in order to achieve a desired temperature profile. In order to grow a single crystal structure a seed of the semiconductor material is brought into contact with the surface of the molten material and then the seed is raised relative to the molten material in order to create the single crystal. During this process it is important that a precisely controlled temperature gradient is achieved within the volume of the heating element in order to achieve a successful crystal growth. Other factors also have to be critically controlled, e.g. seed pull rate, but these will be apparent to the man skilled in the art and are not critical for an understanding of the present invention.
It has also been recognised that applying magnetic fields to the molten material can have a beneficial effect on the crystal growth. When magnetic fields are applied to the molten material, it has also been recognised that there should be zero or near zero magnetic field at the interface of the crystal being grown and the melt material. A known way of achieving such a requirement is to use a magnetic field generating apparatus which comprises two horizontally arranged coils mounted coaxially one above the other and wound such that they are energised in opposite directions so as to produce substantially zero magnetic field at a point on the axis midway between the coils. Then in use the zero point is arranged to coincide with the interface of the crystal and the melt. This is usually achieved by raising or lowering the crucible containing the melt. The coils are also usually contained in a steel enclosure which acts both as a mechanical fixing for the coils and as a magnetic screen.
In order to reduce the power consumption it is known to be advantageous to separate the coils axially to provide an air gap therebetween.